1. Field of the Invention
The present invention relates generally to absolute position detection encoders and, more particularly, to an absolute encoder including a coder provided with a single track or multi-tracks of an absolute pattern graduation scale and at least one track of an incremental pattern graduation scale in parallel each other, and a detector for reading the graduations of the scales. The present invention especially provides an absolute encoder in which a strobe (synchronous) signal giving the timing to read an absolute pattern may be generated at an optional phase position in one pitch of an incremental pattern without mechanically adjusting a position of the detector or a sensor.
2. Description of the Prior Art
An absolute encoder is a measuring instrument which outputs position information of a detector in relation to a graduation on a coder with an absolute position signal inherently adapted to the relative position. The coder and the detector are assembled to allow them to move relative to one another. The coder is provided with an absolute pattern graduation scale having a numerical code of an absolute position signal which is replaced with physical information and arranged in the longitudinal direction of a track. And the detector has a plurality of sensors for discriminating the physical information of the pattern. The absolute encoders are classified into two types according to their appearance, the one is a linear type in which the detector moves relatively and linearly along the longitudinal direction of the strip type coder, and the other is a rotary type in which the detector moves relatively and angularly around a disk or a cylindrical coder. Whichever the type of the absolute encoder may be, it is a measuring instrument which directly reads a graduation information (absolute position signal) registered as physical information on an absolute pattern graduation scale by the sensors of the detector and rearranges into the numerical code by means of an electrical signal processing.
In the past, the common type of the absolute encoders is known as the multi-track type. In this type, an absolute pattern is formed by a plurality of parallel tracks each having an incremental pattern of different pitches, which is called as a multi-track type absolute pattern graduation scale. A detector for reading the multi-track type absolute pattern graduation scale includes a plurality of sensors arranged in the respective positions basically corresponding to that of each track one by one, and an absolute position signal of "binary number of good order" having binary code or gray code is assembled from the outputs of the sensors.
For example, in a four-track type absolute encoder which reads a four-digit binary code, four parallel incremental pattern tracks consisting of mark space patterns of different pitches are formed on the coder, that is:
______________________________________ 2.sup.3 -track ... ... 0000000011111111 2.sup.2 -track ... ... 0000111100001111 2.sup.1 -track ... ... 0011001100110011 2.sup.0 -track ... ... 0101010101010101 Absolute position ... ... defghijklmnop ______________________________________
On the detector, there is arranged four sensors along the direction crossing to the tracks on coder, and their positions at the same phase position of the four tracks are read out in parallel manner so as to obtain sixteen absolute position signals of binary numbers a to p in good order from a=0000, b=0001 to o=1110, p=1111, successively.
Meanwhile, recently a number of absolute encoders using a single-track type absolute pattern have been actively developed and practiced in place of multi-track types. In this type of the encoders, an absolute pattern is constructed with a single track of a mark-space pattern of irregular pitch which is called single-track type absolute pattern graduation scale. In the single-track absolute pattern graduation, "1" and "0" of a special sequence of binary numbers such as Full Periodic Sequence or M-Sequence are replaced by two kinds of minimum reading units having different physical characteristics, and arranged on the track in single line. On the detector, a plurality of sensors are arranged in single line with a pitch fundamentally of the minimum reading unit length along the single-track type absolute pattern graduation scale, and an absolute position signal consisting of "different binary numbers having random order" is assembled from the outputs of said plurality of sensors.
In the absolute encoder using a single-track type absolute pattern, it is extremely advantageous to miniaturize the size of the encoder and to simplify the whole construction of the encoder including their wirings because of the reasons (a) only one absolute pattern is needed, (b) a number of sensors are arranged in single line with a constant distance between them, so that a sensor array is usable in which a number of sensors are collectively formed on one substrate and (c) phase adjustment of sensor position as in case of multi-track type is not needed between each of the tracks.
For example, when using a sequence of numbers 000100110101111 of M-Sequence comprising fifteen binary codes per one period, "0" and "1" of said sequence of numbers are replaced by two kinds of the minimum reading units having different physical characteristics, so as to form a single-track type absolute pattern on the coder, that is:
______________________________________ 000100110101111 abcdefghijklmno ______________________________________
It should be noted that this pattern only indicates one of the minimum unit of repetition, and thus "a" is repeated again after the last "o". In this pattern four succeeding digits are read as a code, and the phase information of each code is represented by the alphabetic character shown below the left end of the four digits. Meanwhile, on the detector are arranged four sensors corresponding to the four succeeding minimum reading units. The sensors generates fifteen absolute position signals comprising binary numbers of random order having a different content of codes each other from a=0001, b=0010 to n=1100, o=1000 one by one. Here, when a practical problem occurs because of the random order of the absolute position signal, the absolute position signal is not allowed to be outputted as it is, but to be converted corresponding to "an address of correct order" such as binary code one to one by using a semiconductor memory in which a proper conversion table is stored, so as to output an absolute position signal of correct order.
Incidentally a single-track type absolute encoder frequently has a malfunction (a misreading of a pattern) when a sensor sweeps the boundary region of the minimum reading unit and therefore, in general, the time to read a single-track type absolute pattern by the sensor is adjusted to read the pattern at a position except for the boundary region.
That is, on the one hand, in the boundary region of the minimum reading unit where the output is inverted, the sensor output becomes unstable, and other hand, there is an inconformity between respective pitchs of the pattern and that of the sensors, and further, the sensor characteristics are not constant, then, there is occasionally a time lag between output change timings in a plurality of sensors. At this time, at least one digit of a binary number which constructs an absolute position signal is outputted as an inverted signal, so that an abnormal absolute position signal which is apart from a correct absolute position may be outputted.
In general, as to the method for adjusting the time to read a pattern, another parallel incremental pattern graduation scale may be used additionally. In this case, the incremental pattern graduation scale is provided on the coder in parallel with a single-track type absolute pattern graduation scale, further, another sensor is added on the detector to detect the incremental pattern, and thus a strobe (synchronous) signal is generated to give the timing to detect the single-track type absolute pattern from the output of the additional sensor.
That is, the position of said additional sensor is mechanically adjusted to the incremental pattern in such a manner that, in the position relationship between the coder and the detector within one pitch of the incremental pattern, if the sensors for detecting the single-track type absolute pattern are on the position where the boundary region of the minimum reading unit is to be detected for example, a strobe signal "0" is outputted from said additional sensor, and if the absolute pattern sensors are on the position to detect a region except for said boundary region, a strobe signal "1" is outputted from the additional sensor, thus the reading of single-track type absolute pattern is performed by the sensors for detecting said absolute pattern only when the strobe signal is "1".
Meanwhile, U.S. Pat. No. 5,068,529 for example, discloses an absolute encoder wherein a set of sensors in the position corresponding to the center of the minimum reading unit is always selected by means of alternatively switching over two pairs of sensor groups one to another in accordance with a strobe pulse obtained from the detection of an incremental pattern in order to secure the timing to read the absolute pattern. In this case, in front of a single-track type absolute pattern of a minimum reading unit length .lambda., said two pairs of sensor groups which comprises a plurality of sensors arranged with a pitch .lambda. are arranged in the detector with a positional phase difference of .lambda./2 between each of the pairs, and thus one of the two pairs of sensor groups is selected depending on "0" or "1" of the incremental pattern of pitch .lambda.. In this way, absolute position signals are successively obtained without any separations on the single-track type absolute pattern, and in any relative position between the coder and the detector, the boundary region is not allowed to be detected.
Incidentally, a similar malfunction may occurr in the multi-track type absolute encoder, the similar measure is used for solving the problem. That is, in a phase position where two or more incremental patterns of different mark-space pitch which construct a multi-track type absolute pattern are inverted simultaneously, if the sensor outputs corresponding to respective patterns are not inverted simultaneously, an abnormal absolute position signal apart from the correct absolute position is also occasionally outputted.
Therefore, in the similar way as above, each pattern except for the last digit is detected by two sensors arranged with a distance corresponding to one half of the minimum pitch (the last digit) of the pattern, and thus, depending on the strobe signal "0" or "1" which is obtained by the detection of the incremental pattern of the last digit, one of the two sensors is to be selected. In this manner, each of the inversion of the respective sensor outputs for all of the patterns will be concurred with each other, so as to make the absolute position signal output stable.
In the above described measures, in which the time period for reading other absolute patterns arranged on the coder is limited by using a strobe signal obtained from the detection of an incremental pattern, and the inversion timings of the sensor outputs corresponding to the absolute pattern are made uniform, the sensor for detecting the incremental pattern should be positioned accurately on the detector. That is, since both the absolute pattern and the incremental pattern are fixedly arranged on the coder so that the mutual phase adjustment is impossible, it is neccessary on the detector side to adjust the positional relation between the sensor for detecting the incremental pattern and the sensors for detecting the absolute pattern within one pitch of the incremental pattern to the accuracy needed for performing each of the above described operation.
For example, in a single-track type absolute encoder where a sensor array for detecting an absolute pattern and an additional sensor for detecting an incremental pattern are preliminarily fixed on the detector, the coder is actually moved relative to the detector, and the mechanical position of the additional sensor is manually adjusted by means of a screw mechanism mounted on the additional sensor while comparing an output from the sensor array with an output from the additional sensor by means of an oscilloscope.
Therefore, it is required for the absolute encoder to hold an additional space for adjusting mechanism of the sensor position and for the adjusting works thereof, thereby the absolute encoder is prevented from miniatuarizing, reducing the number of parts and automization of the assembling process. In addition, the adjusting mechanism is harmful for the solidity of the absolute encoder, and the adjusting works requiring many workers and spending much time make the process rationalization difficult and affect the accuracy and reliability of the absolute encoder. With the recent tendency of high resolution of the coder and reduction of the minimum reading unit length, a higher accuracy is required to the adjusting works, but the accuracy already cannot be attained with the conventional mechanical system.